[39].Surah Al-Zumar [The Groups]: Ayat 55

৫৫. তোমাদের প্রতি অবতীর্ণ উত্তম বিষয়ের অনুসরণ কর তোমাদের কাছে অতর্কিতে ও অজ্ঞাতসারে আযাব আসার পূর্বে,

Ayat 55. "And follow the best of that which is sent down to you from your Lord (i.e. this Qur’an, do what it orders you to do and keep away from what it forbids), before the torment comes on you suddenly while you perceive not!"

Tafseer of Surah Al Zumar Ayat 55. "And follow the Best of (the courses Al-Quran) revealed to you from your Lord, before the Penalty comes on you - of a sudden while ye perceive not! Cf. 39:18. Allah's Command meets the weakness of His weakest servants, and only asks that His servants should surrender their selfish will to Allah's Will. In divine compassion, therefore, we are allowed to do just what we can, even though our standard should fall short of the highest standard, and Allah's Grace will come to our assistance. But we must do so in this life—and at once, as soon as the Word penetrates our mind or understanding. We must not delay a moment, for the Judgment may come at any time, suddenly, before we even perceive where we are.

Hazrat Abu Hurairah (May Allah be pleased with him) reported: The Messenger of Allah [SAWW](PBUH) said,  "If any one of you forgetfully eats or drinks (while observing fasting) he should complete his Saum (fasting), for Allah has fed him and given him to drink.'' [Al-Bukhari Book 03, Chapter 31, Hadith # 154]. 

Lesson: This Hadith tells us of a kindness and convenience of Islam that during the Saum if a person inadvertently does any such thing which destroys his Saum, i.e., eating, drinking, cohabitation with wife, etc., his Saum will remain intact due to forgetfulness, provided one abandons the act as soon as he remembers that he is observing Saum . He needs not make any expiation for such a Saum.